1. Field of the Invention
The present invention relates to starting and driving permanent magnet motors.
2. Description of the Prior Art
Permanent magnet motors are utilized in inertial gyroscope sensing instruments where optimum efficiency, rate stability and low run-up time are required. The derive motors include a two phase stator, a permanent magnet rotor, and suitable drive circuitry to start, run-up and maintain synchronous speed. The rotor is supported by a hydrodynamic bearing wherein the spinning rotor supplies the pressurized gas for the bearing. In the prior art, permanent magnet motor starting is implemented, without feedback, in an open loop manner. Without position feedback wheel starting characteristics are indeterminate and a no-start condition could occur under adverse environmental conditions. In the prior art driving and starting of the motor is achieved via a ramped increasing frequency two-phase drive. At a suitable rotation rate, typically 5% to 10% of synchronous speed, the windings of one phase are opened and its back EMF provides a signal indicative of rotor position and speed. The other phase winding is energized as a function of this signal after the signal has been processed by suitable drive electronics. During normal operation, after starting, the motor is operated single phase.
With the conventional drive technique, the rotor pole locations must be derived from the angular rate of the rotor, and thus the rotor has to be in motion in order to determine it pole position. At rest, zero rate, no back EMF is generated and the pole locations are unknown and the polarity of the drive to the stator phases is indeterminate. Under these conditions, a no start could result or the wheel could be started in the wrong direction would be destructive to the hydrodynamic bearing wheel. Prior art apparatus required complex start/restart electronics to overcome these problems. In addition, in prior art permanent magnet drives, two modes of operation were utilized, a two-phase open loop starting mode and a single phase closed loop running mode.
The present invention eliminates the start/restart electronics and continuously operates in a two-phase closed loop drive mode. The location of the rotor poles is determined optically and is not derived from the angular rotor rate. In addition, both phases are at all times available to be driven for maximum torque.